Date: Tue, 8 Sep 2009 11:52:39 -0300 From: Nahuel Grisolía <nahuel.grisolia@...il.com> To: john-users@...ts.openwall.com Subject: Re: John and RARs or ZIPs Oh! Nice answer! Thanx a lot! Very Educational for the list! Nahuel. 2009/9/8 RB <aoz.syn@...il.com> > On Tue, Sep 8, 2009 at 08:06, Nahuel Grisolía<nahuel.grisolia@...il.com> > wrote: > > Hey Guys, i just want to know if anyone of you has ever coded something > to > > use John to crack the encryption used in RARs (AES128) or ZIPs (??). > > This is a class of query that comes up here quite often - "can JtR be > used to crack X?" The answer depends largely on the implementation, > but is generally "no": JtR doesn't directly support specific file > formats, it handles password hashes. > > In clarification (and simplified terms), you need to understand the > difference between hashing and encryption. Hashing is a one-way > process by which a statistically unique small value is computationally > derived from a [typically] larger data set. The "one-way" part is > critical: you cannot derive the original data from a hash, only repeat > the process and confirm you have a precise copy. Encryption is a > bidirectional process by which data may be converted to and from an > opaque form by use of a secret key. In well-designed systems, > passwords are stored in hashed form - you cannot derive the original > data directly from the hash, but you may hash the data you have and > compare the two. What JtR does is create a list of potential > passwords, hash them, and then compare them against a specified hash > until it finds a match (or reaches the heat death of the universe), > hence: > > John the Ripper does perform any decryption. > > I split that out because it's incredibly important to understanding > the role of password "crackers". Password crackers generally do not > do any decryption, they only bumble along (some more intelligently and > quickly than others) trying to make up something that matches the > known hash. > > If a particular file format were to be so generous as to include a > hash of the password, it would be possible to extract that hash and, > if necessary, create a JtR handler for the hash form. However, most > encrypted formats aren't so poorly designed and will happily decrypt > with whatever key they're given, forcing the attacker to evaluate > whether the decrypted results are valid. That evaluation is beyond > the scope of an application such as JtR. > > > How can we know if the RAR or ZIP is really desencrypted? > > The formats probably have internal checksums or well-known values that > are checked post-decryption to allow validation of the key. > > >
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